For half a century, social psychologists have been trying to figure out the
human gift for rationalizing irrational behavior. Why did we evolve with brains
that salute our shrewdness for buying the neon yellow car with bad gas mileage?
The brain keeps sending one message -- Yesss! Genius! -- while our friends and family are saying,

"Well... "

This self-delusion, the result of what's called cognitive dissonance, has been demonstrated over and over by researchers who have come up with increasingly elaborate explanations for it.
Psychologists have suggested we hone our skills of rationalization in order to impress others, reaffirm our "moral integrity" and protect our "self-concept" and feeling of "global self-worth."

If so, capuchin monkeys are a lot more complicated than we thought. Or, we're less complicated. In a paper in Psychological Science, researchers at Yale
report finding the first evidence of cognitive dissonance in monkeys and in a group in some ways even less sophisticated, 4-year-old humans.